1. Field of the Invention
The present invention relates generally to the field of printed circuit boards (xe2x80x9cPCBsxe2x80x9d). More particularly, the present invention relates to a method of making higher impedance traces on a low impedance circuit board.
2. Discussion of Related Art
The common types of PCBs are a double-sided PCB and multi-layered PCB. A double-sided PCB includes conductive planes formed on the both sides of an insulation layer. A multi-layered PCB includes a plurality of conductive planes and insulation layers. In a multi-layered PCB, an insulation layer is typically formed in between conductive planes. The multi-layered PCB can have three or more conductive planes. The conductive planes refer to signal trace layers, power planes, or ground planes.
FIG. 1A and FIG. 1B show a multi-layered PCB 100. FIG. 1A shows an illustration of a cross-sectional view of PCB 100. FIG. 1B shows an illustration of a top view of PCB 100 of FIG. 1A.
Referring to FIG. 1A, PCB 100 includes signal traces 190, 193, 195, and 197 and conductive planes 110 and 155. PCB 100 also includes insulation layers 105, 130, 140, and 160 formed in between the signal traces and conductive planes, a blind via 102, and a buried via 103.
Signal traces 195 and 197 are formed on a bottom side of insulation layer 105. Blind via 102 is formed in insulation layer 105. Conductive reference plane 110 is formed on insulation layer 105 and over blind via 102. Blind via 102 is a plated-through hole, which can be used to couple electrically conductive reference plane 110 with signal trace 197. Insulation layer 130 is formed on conductive reference plane 110. Buried via 103 and signal traces 193 are formed in insulation layer 130. Buried via 103 is a plated-through hole, which can be used to couple electrically one of the signal traces 193 with conductive reference plane 110. Insulation layer 140 is formed on insulation layer 130 and signal traces 193. Signal traces 195 and 197 can be used to interconnect electronic components (not shown) on the bottom side of insulation layer 105. Signal traces 193 can be used to interconnect electronic components on insulation layer 130. Signal traces 190 can be used to interconnect electronic components on insulation layer 160. As shown in FIG. 1B, signal traces 190 can have varying shapes and sizes.
Insulation layer 105 provides insulation between signal traces 195 and 197 and conductive reference plane 110. Insulation layer 130 and 140 provide insulation between conductive reference planes 110 and 155. Insulation layer 160 provides insulation between conductive reference plane 155 and signal traces 190. Conductive reference plane 155 is a ground plane, which can be used as a common electrical circuit return. Conductive reference plane 110 is a power plane, which can be used to provide specified potential to the signal traces.
In recent years, double-sided and multi-layered PCBs have become increasingly thinner to meet the demand of consumers for smaller and more compact electronic products. One way used to make thinner PCBs is by reducing the thickness of the insulation layers between the conductive planes. However, reducing the thickness of the insulation layers of the signal traces can affect the characteristic impedance of the signal traces on the PCBs.
The characteristic impedance of a signal trace is primarily determined by inductance and capacitance as shown in Equation (1):                               Z          0                =                              L            C                                              (        1        )            
in which Z0 is the characteristic impedance of the signal trace, L is the inductance per unit length of the signal trace, and C is the capacitance per unit length of the signal trace. Furthermore, the capacitance per unit length of the signal trace is generally expressed as shown in Equation (2)                     C        =                  KS          d                                    (        2        )            
in which C is the capacitance per length of the signal trace, K is the dielectric constant, S is the electrode plate size (primarily width of the signal trace), and d is the distance between two electrode plates (the separation distance between the signal trace and the nearest conductive plane).
When these two equations are combined, the resulting equation is as shown in Equation (3)                               Z          0                =                              Ld            KS                                              (        3        )            
According to Equation (3), if the inductance per unit length of the signal trace (L), dielectric constant (K), and the width of the signal trace (S) remain constant, the characteristic impedance of the signal trace can decrease by decreasing d, which is the separation distance between the signal trace and the nearest conductive plane.
Typically, the reduction of the separation distance is beneficial because such reduction reduces cross-talk and lessens the effects of electromagnetic interference (xe2x80x9cEMIxe2x80x9d) on the signal traces. However, in certain applications, some signal traces, such as video signal traces, require higher impedances to match properly with electronic components, such as video displays, that operate with higher impedances.
According to Equation (3), the characteristic impedance of a signal trace can be increased by keeping the factors L, K, and S constant and increasing d, which is the separation distance between the signal trace and the conductive reference plane that is located closest to the signal trace. This, however, increases the thickness of an insulation layer thereby causing the characteristic impedance of all other signal traces on the insulation layer to be increased.
Another way to increase the characteristic impedance of a signal trace, according to Equation (3), is to decrease the width of the signal trace (S). However, decreasing the width of the signal trace may significantly increase the cost of fabricating a PCB and may violate manufacturing standards.